Germanium has a direct band gap E0=0.81 eV at room temperature (see, MacFarlane and Roberts, Phys. Rev. 97, 1714 (1955)), which corresponds to an optical wavelength of 1.54 μm. Although this is barely enough to reach the telecom C-band (1.53 μm-1.56 μm), the very strong wavelength dependence of the absorption coefficient near the direct edge suggests that small perturbations that shift E0 to lower energies should dramatically improve the performance of this material for use in optical fiber communications. The direct band gap of Ge can be reduced by alloying with Sn and by applying stress. The dependence of E0 on Sn-concentration has been measured and found to be stronger than predicted, to the extent that the addition of only 2% of Sn increases the absorption coefficient at 1.55 μm by more than one order of magnitude (see, D'Costa et al., Phys. Rev. B 73, 125207 (2006)). The use of stress as a perturbation is problematic because the direct band gap can only be lowered with tensile strain.
This strain cannot be obtained by growing epitaxial Ge on Ge1-xSix alloys because the latter have a smaller lattice parameter. In spite of this inherent limitation, tensile-strained Ge has been obtained by depositing the material directly on Si at relatively high temperatures and by exploiting the smaller thermal expansion of the substrate to induce stress in the Ge epilayer when the sample is quenched from ˜800° C. (see, Ishikawa et al., Appl. Phys. Lett. 82, 2044 (2003); Cannon, et al., Appl. Phys. Lett. 84, 906 (2004); Liu et al., Appl. Phys. Lett. 87, 103501 (2005); Wietler et al., Thin Solid Films 508, 6 (2006); and Liu et al., Phys. Rev. B 70, 155309 (2004)). This process leads to biaxial tensile strains as high as 0.25% in films as thick as 1 μm. While such tensile strain values may be sufficient for some photodetector designs, higher strain values are necessary for most optoelectronic applications which require tunable direct gaps (see, Liu et al., 2004, supra).
Further limitations of the thermal expansion process include lack of precise strain control and a maximum predicted strain value of 0.3% for growth at 900° C. (see, Cannon, et al., supra). Moreover, the use of high temperatures (800-900° C.) typically induces inter-diffusion of the elements across the Si-Ge heterojunction, resulting in non-uniform and potentially defective interfaces. In the context of laser applications, spatial confinement requires abrupt interfaces, which are precluded using this high temperature process due to the inherent elemental intermixing at the interface. In addition, precise and systematic control of the final strain state has not been demonstrated using this method, and this hampers the design of devices.
Thus, there exists a need in the art for improved methods of preparing tensile strained Ge on semiconductor substrates.